Pimavanserin tartrate, 1-(4-fluorobenzyl)-3-(4-isobutoxybenzyl)-1-(1-methylpiperidin-4-yl)urea L-hemi-tartrate, has the following chemical structure:

Pimavanserin tartrate was developed by Acadia Pharmaceuticals and was approved under the trade name NUPLAZID® for use in patients with Parkinson's disease psychosis.
Pimavanserin free base and its synthesis are disclosed in U.S. Pat. No. 7,601,740 (referred to herein as US '740 or the '740 patent) and U.S. Pat. No. 7,790,899 (referred to herein as US '899 or the '899 patent). US '740 discloses the synthesis of Pimavanserin free base (also referred to herein as “Compound A”), which includes O-alkylation followed by ester hydrolysis, and then in situ azidation. This process suffers from low process safety, and utilizes the hazardous reagent diphenylphosphoryl azide. The process is illustrated by the following Scheme 1.

US '899 describes another process, which includes O-alkylation followed by aldehyde reductive amination to obtain an intermediate which is then reacted with the hazardous reagent phosgene. This process is illustrated by the following Scheme 2:

Both of the above processes for the preparation of Pimavanserin include a reaction between 1-isobutoxy-4-(isocyanatomethyl)benzene, a benzyl isocyanate intermediate, and N-(4-fluorobenzyl)-1-methylpiperidin-4-amine. Processes for preparing benzyl isocyanate derivatives are generally described in the literature, such as in US '740; US '899; Bioorganic & Medicinal Chemistry, 21(11), 2960-2967, 2013; JP 2013087107; Synthesis (12), 1955-1958, 2005; and Turkish Journal of Chemistry, 31(1), 35-43, 2007. These processes often use the hazardous reagents like phosgene derivatives or diphenylphosphoryl azide.
Accordingly, there is a need for additional processes allowing the efficient and safe synthesis of Pimavanserin.